30 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
shews the extreme liquidity of the stream, which must have shot through this 
narrow and tortuous valley with a rapidity truly astonishing, leaving its scum 
and slag to mark the height which it attained; but the main flood of lava not 
having time to solidify, was propelled ever onwards, leaving often but a narrow 
thread in the channel of the stream, by which to trace its passage. 
One circumstance struck me very forcibly as regards the mineral character 
of this lava. It contains most abundant nodules of olivine, which have not in the 
least the appearance of being formed im the lava by a slow crystallizing process ; 
but, on the contrary, the irregularity of their forms is decidedly fragmentary, often 
as if rolled ; and from the manifest impurities which they present, I am of opinion 
that they are nothing but fragments of granite in a peculiar metamorphic condi- 
tion. This idea first struck me several years before I visited the Vivarais, in an 
examination of the lavas of Clermont; where I found olivine masses near La 
Barraque, having at first sight the aspect of granite fragments, but when examined, 
they appear to be only olivine, with some adhering reddish micaceous and clayey 
matter.* The olivine of Burzet presents two varieties of a pale yellow green, and 
of a peculiar orange colour. They have no concretionary structure depending 
on their form. Similar phenomena are common in different parts of the Vivarais, 
but less, I think, in lavas which are decidedly columnar. - The frequency of ap- 
parently ejected masses of almost pure olivine in many volcanic countries, and 
most especially in Upper Eyffel (as at the crater of Dreiser-Weyer) gives an ad- 
ditional probability to the hypothesis that they are foreign substances expelled in 
an altered condition. Long after making these remarks, I noticed a passage in Mr 
ScroPe’s work on Central France, tending to the same conclusion; namely, that 
these olivine nodules are altered masses of pinite from the granite. + 
In 1839 I pursued the traces of the lava of Burzet about 4 miles beyond 
the village of that name, or at least 8 from the opening of the valley, without 
seeing a trace of a crater; and my time did not then allow me to prosecute the 
search. But it was one of the objects of my second visit in 1841 to resume it; 
and, accordingly, accompanied by my friend Mr Joun Macxintosu, I slept 
at Burzet in very uncomfortable quarters, in order to have the whole day for 
our excursion, resolving to ascend as high as the great ridge which separates the 
waters of the Ardéche and of the Erieux. We then readily found the point 
which I had before reached, where the lava temporarily disappears from the 
valley, though there is no appearance of a crater; and this disappearance con- 
* M. de Bucn in his early writings (Journal des Mines, XIII., p. 251) notices the olivine of La 
Barraque, without saying anything as to its origin, except that he considers it an exclusive character 
of old basalt, as contrasted with lavas, at least in that country. 
+ Scrorz, p. 150. Favsas Sr Fonp speaks of the lava at Beaume containing “ petits éclats de 
granit en chrysolite,” which seems to indicate a similar opinion. (Recherches, p. 300.) He also 
describes masses of olivine existing in the basalt of the valley of Burzet, in the meadows below the 
village of St Pierre Colombier, weighing as much as 30 pounds. This remarkable statement deserves 
verification. See Recherches sur les Volcans eteints, pp. 249 and 312. 
